The electrically driven vehicle is capable of traveling by conveying the driving force of a motor/generator to wheels, and is further capable of braking the wheels by a cooperative control between regenerative braking due to an electric generation associated load of a motor/generator and friction braking due to the hydraulic brake unit when required. The electric power that is generated by the motor/generator is stored or charged in a battery for use as electric power during motor drive.
Incidentally, when performing brake operations on the wheels by the cooperation between the regenerative braking and frictional braking, in the conventional cooperation braking system, in general, a priority is given to the regenerative braking in terms of an energy recovery rate, and when braking the wheels by just regenerative braking is insufficient to achieve the target braking torque required by the driver, the shortage will be compensated for by the friction braking.
Therefore, when attempting to start the electrically driven vehicle on such as uphill road by releasing a brake pedal and depressing on an accelerator pedal, the vehicle may rolls back in the direction opposite to the start up or starting direction due to the road gradient. To control and prevent this rollback at start up, as described in Japanese Laid-Open Patent Application Publication No. 2007-203975, for example, the wheels will be braked via regenerative braking to prevent the vehicle rollback.
However, if the power source battery is in a fully charged or a nearly fully charged state, or the battery is restricted to be charged at extremely low temperature, the motor/generator is prevented from generating electricity for performing the regenerative braking at the wheels.
In this case, the motor/generator cannot output the driving force by being operable as a generator, the electrically driven vehicle rolls back continuously in the reverse direction of the desired direction of the driver despite trying to start the vehicle by depression of the accelerator pedal after the release of the brake pedal, there arises a problem of giving a sense of discomfort to the driver.